We have isolated 14 temperature-sensitive (ts) mutants of Sendai virus which fall into two groups: one mutant synthesizes RNA but not infectious virus (RNA plus), the remaining mutants synthesize neither RNA nor infectious virus (RNA minus). To determine the defects in RNA minus mutants, we will study virion-associated RNA transcriptase, formation of transcriptive and replicative intermediates, and the assembly and functions of transcriptive complexes. All mutants (RNA plus and RNA minus) will be examined for synthesis of single-stranded RNAs and proteins. The RNA plus mutant has a ts hemagglutinin. This mutant will be examined for other changes in surface properties, including neuraminidase activity, hemolysis and cell fusion, and for defects in virus assembly. Viral macromolecules will be analyzed by rate-zonal and isopycnic centrifugation, polyacrylamide gel electrophoresis and RNA:RNA hybridization.